The present invention relates to a semiconductor device and, more particularly, to a technology which is effective if applied to a semiconductor device having a capacitor element.
In a semiconductor device, especially a semiconductor device including a memory cell of DRAM (Dynamic Random Access Memory) having a selecting MISFET and a capacitor element connected in series, the size of the memory cell has been reduced to improve the degree of integration. Accordingly, the size of the capacitor element is also reduced. In order to keep a reliability to soft errors due to alpha rays, however, the amount of charge to be stored by the capacitor element cannot be decreased less than a predetermined value. Thus, a capacitor element of stacked structure is used to reduce the size of the memory cell and to improve the reliability to the soft errors.
The capacitor element is composed of a lower electrode and an upper electrode formed over the lower electrode through a charge storing insulating film. The lower electrode is formed of a polycrystalline silicon film. The lower electrode is connected with the source or drain region of the selecting MISFET. The charge storing insulating film is formed of a silicon oxide film which is prepared by thermally oxidizing the polycrystalline silicon film forming the lower electrode. Alternatively, the charge storing insulating film is formed of a laminated film which is composed of a silicon nitride film formed over the polycrystalline silicon film and a silicon oxide film formed over the silicon nitride film by thermally oxidizing the silicon nitride film. The upper electrode is formed of a polycrystalline silicon film.
In order to further reduce the size of the memory cell, however, the size of the capacitor element has to be reduced by increasing the dielectric constant of charge storing insulating film thereby to increase the amount of charge to be stored per unit area. Thus, there has been proposed a method by which the silicon oxide film and the silicon nitride film are replaced by a metal oxide film such as a tantalum oxide (Ta.sub.2 O.sub.5) film having a higher dielectric constant than those of the silicon oxide film and the silicon nitride film so as to form the charge storing insulating film.
The tantalum oxide film is prepared by the CVD method, for example. The technology of this kind is disclosed, for example, on pp. 13-14 of IEEE, Symposium on VLSI Technology (1990).
On the other hand, the tantalum oxide film is prepared, for example, by forming a tantalum film by a the electron beam evaporation method or the high-frequency sputtering method and subsequently by oxidizing the tantalum film. The technology of this kind is disclosed, for example, on pp. 6502-6508 of American Institute of Physics, Journal of Applied Physics, 54(11) (1983).
According to the method of preparing the tantalum oxide film by forming the tantalum film by the electron beam evaporation method or the high-frequency sputtering method and subsequently by thermally oxidizing the tantalum film, however, this tantalum film has a rough surface, when it is formed, so that the thickness of the tantalum oxide film formed by the thermal oxidation will disperse. With this dispersion in the film thickness, the breakdown voltage of the tantalum oxide film will drop to raise a problem that the reliability of the semiconductor device degrades.
Thus, there has been proposed a method by which the tantalum oxide film formed by the reactive sputtering method is oxidized again. Thanks to this oxidation, the underlying silicon of the tantalum oxide film is thermally oxidized in the region, in which the tantalum oxide film is thin, to form the silicon oxide film. According to this method, the breakdown voltage of the portion having the thin tantalum oxide film is improved by the silicon oxide film formed by the oxidation step so that the breakdown voltage of the charge storing insulating film can be improved. As a result, the reliability of the semiconductor device can be improved. The technology of this kind is disclosed, for example, on pp. 1622 to 1628 of Applied Physics of Association of Applied physics 58, 11 (1989).